Two numbers on a drawing — "40-20", "60-40" — decide whether a finished germanium window ships or gets scrapped. They come from MIL-PRF-13830B, "Optical Components for Fire Control Instruments; General Specification Governing the Manufacture, Assembly, and Inspection of" — issued 9 January 1997 as the successor to MIL-O-13830A of 11 September 1963, a lineage that starts in the 1950s and has been revalidated by Notices in 2000 and 2013. Half the industry reads those numbers wrong. Here is how an inspector reads them.
The micron myth
The scratch number — 10, 20, 40, 60, 80 — is not a width in microns. It is a grade of visibility: the scratch on your part is compared, under controlled lighting, against physical reference standards (Drawing C7641866, with prototypes maintained at Picatinny Arsenal). The myth has a real origin: the 1954 text suggested microns, and the 1956 revision changed the wording to "tenths of a micron" — but the physical standards never changed. A scratch "width" conversion chart is a fiction; the standard is a brightness comparison, full stop.
The dig number, by contrast, is dimensional: the actual defect diameter in hundredths of a millimeter. Dig 40 = 0.40 mm; dig 20 = 0.20 mm.
| Accumulation Rules (MIL-PRF-13830B) | |
|---|---|
| Maximum-grade scratches | Combined length ≤ ¼ of the element diameter |
| Maximum-size digs | 1 permitted per 20 mm of diameter |
| Digs < 2.5 µm | Ignored |
| Digs grade ≤ 10 | Must be separated by ≥ 1 mm |
| Inspection Methods | |
|---|---|
| Method 1 | 40 W incandescent / 15 W fluorescent behind frosted glass |
| Method 2 | Scattered light viewed at ~90° against a dark background |
MIL vs ISO: two languages, no dictionary
ISO 10110-7 states surface imperfections as 5/N×A, where A is the square root of the defect area in millimeters — a purely dimensional system. Because MIL scratch numbers are visibility grades, there is no exact conversion between the two. Practical equivalences exist (Aikens): scratch #40 ≈ L 2×0.010; #60 ≈ L 3×0.010 — useful for translating drawings, dangerous if treated as physics.
What to put on the drawing
40-20 is the industry's workhorse — commercial ZnSe CO₂ laser windows are routinely 40-20. Step up to 20-10 for precision laser surfaces, and 10-5 only for intra-cavity optics, where every defect sits inside a resonator that revisits it thousands of times per second. Beyond that, tighter scratch-dig is money spent on cosmetics: on Ge thermal-imaging optics far from a focal plane, a 60-40 surface rarely moves the MTF at all.